I had problems after replacing the hose and barbs following a breakage.
Turned out that I hadn't wound the barb into the hose far enough at the seat end and it was weeping slightly when activated, then presumably allowing air into the system upon "return".
Hard to spot because the replacement seat barb is shrouded for protection.
Once I'd wound it in a bit tighter and re-bled, it was fine.
The actual bleed itself is very staightforward and hard to get wrong.

Agreed. Exactly the same symptoms. First actuation reasonably good, then deteriorating rapidly thereafter.
Easily sorted.
Turned out the hose needed to be threaded quite a bit further onto the barb.
The "stop" point at the end of the thread was quite noticable in the end.